What you can already do is to attach a function that is executed when the preload state changes so you can implement custom sample preloading bars. If you just want to react on user preset changes, take a look at the solution posted in the other thread.

But its a good idea and every method that returns void can be changed to return a error code. However its a bit unclear what a "success" in this case is:

Does the file exist?

Is it a valid XML file?

Does the version match or does it contain any controls that aren't part of the current instrument or have the wrong type?

The more you go down this list, the more stuff it has to do synchronously and at some point it looses the advantages of an async call (which is instant returning).

@d-healey yes I know I can.... but that doesnt meet the criteria of "simple" I think...

JavaScript, on which HISE script is based, doesn't have a wait/sleep type thing and I don't think it's a good idea to start tacking on bits of KSP to solve a problem that doesn't really exist. I agree that writing less lines of code = simpler but I'm not sure the difference in level of simplicity is as important as the functionality which is already there.